What is Negative Voltage?

Negative voltage in a circuit is voltage that is more negative in polarity than the
ground of the circuit.

A voltage source has positive or negative polarity depending on its orientation in a circuit. In the case when
a voltage source has negative voltage, it just means
the negative terminal of the battery is connected to the positive side of the circuit and the positive terminal of the battery is connected
to the negative side of the circuit .

Look at the diagram below for an illustration of positive and negative voltage:

Both voltage sources are connected to an LED. The first circuit is oriented one way and the other circuit is oriented the same way but with the voltage source flipped around, so
that is has opposite polarity orientation than the first. The first circuit is positive voltage because the positive end of the voltage is connected to the positive end of the source is not
connected to ground, while the negative end of the voltage source is connected to ground. The second circuit is the exact opposite, where the positive end of the voltage source is connected
to ground, while the negative end is not.

As you are probably familiar, an LED is a device that only turns when the anode of the LED is supplied to positive voltage and the cathode with negative voltage. This is why
only the first circuit's LED turns on, while the second circuit's LED doesn't.

To further be able to understand this, think of when you put a battery in a device. If you have a battery inserted the correct way in a circuit, it normally is in its positive orientation
and it supplies positive voltage to the circuit.
It will produce a current through the positive part of the circuit and then go down to ground and back to the negative part
of the battery, and the cycle goes on over and over again. However, if you now flip the battery around to its other end, it
will now have the opposite orientation, negative
orientation, in the circuit. If a battery is inserted in a circuit with its positive orientation in which it produces
current to the positive part of a circuit
and it's now flipped around to the other side, it will now produce current to the ground of the circuit instead of the positive
part of the circuit. This is all negative voltage means, a polarity switch of the battery from its positive orientation
to its opposite side, its negative orientation.

This is what happens when you insert batteries into a nintendo gameboy. If you put in
live batteries the correct way, the gameboy will have power
and it can be turned on. But when you put in the batteries the wrong way, it will not start and power on. When batteries are put in
the correct way, the voltage source delivers current to the positive part of the circuit, which the circuit needs to turn on.
However, when batteries are put in on the wrong side, with negative polarity, the voltage source delivers current the other way,
to the ground of the circuit rather the positive part. Current is not useful this way, and if the circuit isn't equipped with
protective diodes, in fact, it could destroy parts of the circuit. Being that the voltage source delivers power to the ground
rather than the
positive part of the circuit which needs power, the circuit will not turn on.

A way to measure the voltage orientation in a circuit is by using a multimeter placed on the voltmeter settings. Place the ground
test lead of the multimeter on the side of the battery that touches ground or any part of ground in the circuit, since they're
all common, and place the positive test lead of the multimeter on the lead of the battery that makes contact with the positive side
of the circuit. By doing this, you can tell whether the battery has positive or negative orientation in a circuit. If a battery
has positive orientation, its voltage with respect to ground is more positive than ground, and it delivers current to the positive part
of a circuit. If a battery has negative orientation, its voltage with respect to ground is more negative than ground, and it delivers
positive current to the ground part of a circuit.

Uses of Negative Voltage

Negative voltage is not always unintended. Yes, if you switch the orientation of a voltage source to negative polarity for many circuits, the circuit may not work
anymore, but in other circuits, negative
voltage has extreme use and cannot work.

One example of an electronic device that many times uses negative voltage is a transistor. When a transistor is biased many times, negative voltage is placed on the transistor's
emitter lead. This type of bias is called emitter bias, shown below:

When negative voltage is placed on the transistor's emitter lead, the electrons in an (NPN) transistor's emitter are repulsed from the negative voltage. Remember that in a BJT
transistor, current flows out from emitter to collector. Now that the negative voltage repulses the electrons in the emitter and, thus, forces electrons to the collector, it
helps to push current out from the emitter to collector.

Another such device that uses negative voltage is an op amp circuit, since it's composed of many transistors.
Many op amps function off of a dual power supply. One lead of the op amp receives positive voltage and the other receives negative voltage.